


An Experiment

by ElAlmaDelMar



Series: A/B/O/tober 2020 [20]
Category: One Piece
Genre: Alpha Luffy, Alpha/Beta/Omega Dynamics, M/M, Objectification, Omega Topping, Size Difference, Size Kink, Stomach Bulge, This is pre-TS so Luffy is underage, not like it's a huge diff. 17 and 19 years old. but still., omega zoro, omega/alpha, person used as a sex toy, temporary size difference
Language: English
Status: Completed
Published: 2020-10-20
Updated: 2020-10-20
Packaged: 2021-03-09 07:00:52
Rating: Explicit
Warnings: Underage
Chapters: 1
Words: 2,694
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/27129607
Author URL: https://archiveofourown.org/users/ElAlmaDelMar/pseuds/ElAlmaDelMar
Summary: Zoro woke up to a warm, squirmy weight depositing itself in his lap."Hey, Zoro," Luffy said, settling himself comfortably with his back against Zoro's chest.  "I got an idea."A/B/O/tober Day 20: Size difference
Relationships: Monkey D. Luffy/Roronoa Zoro
Series: A/B/O/tober 2020 [20]
Series URL: https://archiveofourown.org/series/1947415
Comments: 4
Kudos: 95
Collections: A/B/O/tober 2020





	An Experiment

Zoro woke up to a warm, squirmy weight depositing itself in his lap. 

"Hey, Zoro," Luffy said, settling himself comfortably with his back against Zoro's chest. "I got an idea." 

That pronouncement didn't meet with an immediate response; Zoro took a moment to loop one arm loosely around Luffy's waist and rest his chin on Luffy's shoulder. When Luffy said _idea_ like that, he didn't mean something like _let's take a nap together on our new ship's grass deck_ or anything as restful. Whatever was going to follow was going to be some form of exciting. 

Only when he'd taken his moment to enjoy his rubbery captain's presence did he respond. "What kind of idea?" 

Luffy didn't seem at all bothered by the delay; Zoro hadn't thought he would be. He was highly tactile, far more so than most alphas, and cuddled with every one of their nakama on a regular basis, scenting them and cementing the bonds of affection and loyalty that held their little pack-crew together. So Zoro taking a little extra time to really sink into the feeling of Luffy filling up his lap was just expected. That was how they did things. 

"So, you know how I got really tiny after using Gear Third?" Zoro couldn't see Luffy's face in this position, but he could hear the grin. "We should have sex like that." 

"We- _Luffy_." The name came out halfway between a choke and a sigh of pure exasperation. "Where the hell did that come from?" 

"I dunno. It just seems like fun? Besides, I bet your dick would feel _huge_ like that." Luffy twisted his head to look over his shoulder at Zoro, grinning brightly, and Zoro scowled at him in return. 

"Are you saying I ain't big enough for you normally?" It was more than half a joke — he knew full well that Luffy enjoyed him just as much as he enjoyed Luffy, no matter what they were getting up to in bed — but at the same time, omega stereotypes were hard to dodge. 

"Shishishi, are you worried?" The words came punctuated with a light kiss to Zoro's temple. "Zoro takes good care of me. But I wanna feel what it's like when I'm tiny. Can we?" 

The idea was… weirdly intriguing, if Zoro let himself think about it, but between the two of them _someone_ had to actually think things through. "Doesn't that shit fuck with your bones?" he asked, frowning. "You're not doing that to yourself just to try some weird shit in bed." 

Luffy pouted, but didn't argue. "Okay, fine. The next time we get in a big fight?" 

"Maybe." Zoro wasn't about to commit to anything more than that. "We'll see."

* * *

Well, it looked like Luffy was going to get his wish, courtesy of another pirate crew that attacked them on an island stop. 

Most of the crew were pretty unremarkable, and Zoro and Sanji between them cleaned up the so-called officers of the Mighty Fist Pirates — but their captain was something else, and Luffy had a hell of a time of it before the guy went down. The guy looked to have some giant blood in him or something, towering over everything in his vicinity, and packed a commensurate punch. Not that it was going to stop Straw Hat Luffy, even so! And once his opponent was face-down in the dirt and Luffy stood the victor, he released Gear Third and popped down to that so-tiny, portable size. 

"Right!" Zoro, the closest one on hand, scooped up the tiny Luffy and looked to the rest of the crew. "Let's head out before these guys wake up for round two. Nami, did the Log Pose set yet?" 

"It just did," she confirmed, nodding. 

"Good. We're outta here." 

It had been a long fight; Luffy wasn't back to normal by the time _Sunny_ rolled out on the tide. And when he grabbed Zoro's pant leg and tugged with a determined bright light in his eye, Zoro knew exactly what was going on in his empty little head. 

He hadn't forgotten Luffy's idea. Had thought about it kind of a lot, in fact. Their captain was a lot less stretchy when he was like this; he'd be tight as hell, but Zoro'd have to worry about hurting him — not something that was normally a concern when Luffy wanted to ride him, no matter that alphas weren't really designed to take it. 

Franky had built _Sunny_ with an intentional cycle room — a huge step up from the spare futon stashed in one of _Merry_ storage rooms. And in between cycles — well, they were far from the only ones using the room for more temporary privacy. 

"You sure about this, captain?" he rumbled as he closed the door behind the two of them, isolating them down in the privacy of the room. "I'd think this'd be the kind of time you should be stuffing your face and taking a nap." 

"I'll do that after," Luffy said confidently. "I wanna do this first though. It's gonna be fun." He had to clamber up onto the bed, then patted the edge of it determinedly. " _Come here_ , Zoro." 

Shit, it really did something to Zoro when Luffy used that tone of voice — it wasn't quite classic commanding alpha, but close, and there was something warmer, more intimate to it than that. Not an alpha ordering an omega, but a pack leader asking for his first nakama's cooperation. Strong without being forceful, that was it. 

He settled himself on the edge of the bed, and Luffy promptly climbed into his lap and tugged his head down for a hungry kiss. _Someone_ was already getting excited for their little experiment here! 

Not that Zoro intended to let him down. His hands on Luffy were gentler and more careful than usual as he returned that kiss, supporting Luffy to make sure he wouldn't fall backward. He was gonna have to be the one thinking about shit like that, because Luffy — already tugging on the collar of his shirt, too small to properly undress Zoro himself — sure as hell wouldn't. 

It took a little bit for Zoro to fall into the rhythm of things. He was so accustomed to Luffy being all over him — being a sprawl of long, bendy limbs, wrapping around him and touching him everywhere — that the difference was startling. Luffy had no reach right now, felt even smaller and more confined than the change in his physical size suggested. But he still smelled like Zoro's alpha, and scent meant the most. Zoro could still knock his hat back and nuzzle into dark hair, breathing in his scent and letting it suffuse his entire awareness. 

Luffy always smelled so _good_. Like home, happiness, pack. And right now, like an alpha who dearly wanted some attention. 

Attention Zoro wouldn't be denying him. He stripped Luffy's clothes off him — weird, how they shrank right along with Luffy when he went like this, but right now was not the time to question that — slowly, letting his hands linger over warm skin, touching and exploring this new tiny version of his captain. His limbs were even shorter yet in proportion to the rest of him, his whole body collapsed in on itself to rebuild. His fingers circled tiny nipples, and Luffy shuddered at the scrape of callus — teased at the little pockmark of a navel — and slid his hand down to cup between Luffy's legs and stroke him. 

"Heh. Tiniest knot in the Grand Line," he teased, nipping at Luffy's ear, and Luffy laughed, unbothered. 

When Zoro's clothes joined Luffy's on the floor, Luffy tried to shove him onto his back on the bed — and found Zoro unmoved. "Gotta use your words now, captain," Zoro told him, grinning. "Can't just throw me around anymore." 

"Fiiiine. Zoro, lie back?" Luffy tried for the wide-eyed cute and appealing look instead — it didn't quite work, not when he had a flushed little cock trying for attention, but Zoro wasn't here to argue — he just sprawled back on the bed and let Luffy do as he would. 

"Whoa." It took both of Luffy's hands to wrap around his shaft, and his fingertips weren't even meeting when he did. "Zoro, you look _huge_ like this!" 

"Feels funny," Zoro admitted as Luffy tried to figure out how to stroke him properly at their disparate sizes. It was a whole-arm motion, and on Zoro's end, the press of two tiny hands felt — different. Strange. Like nothing he'd had before. 

Kind of good, now that he thought about it. He hadn't considered this part — how much the novelty factor alone would do for him. But he was fully hard, and as Luffy's little fingers explored him, almost _tickling_ in their touch, he could see a bead of pre gathering at his tip. 

"You like this, don't'cha?" Luffy challenged, grinning, and stretched up to lick away that little bead. "Zoro's having fu~un!" 

"Yeah, it's always a fun time with you," Zoro agreed, rolling his hips upward into Luffy's touch. A thought occurred to him, and he grinned, showing a little fang. "... Betcha can't fit any of it in your mouth now, captain." 

"Ha! You're on!" Luffy grinned wide and sucked the tip in, closing his lips around the flared head with some obvious difficulty. Now _that_ was new — and so was how tight his mouth felt, the press of palate and tongue immediate. Normally a cock was nothing compared to the vast stretch of Luffy's mouth, but tiny _and_ far less stretchy than usual? There was something kind of fun about that… 

Of course, it meant Zoro couldn't fuck Luffy's mouth with total abandon the way he usually would. In fact, he resisted the temptation to move at all — just rested a hand lightly against the back of Luffy's head and let him set the pace, the combination of hands stroking and mouth sucking gently. 

It was good. It was damn good. He was pretty sure he could come from this alone…. But that wasn't what Luffy had asked for, now was it? "Mmn… you sure you wanna take this farther? Feels pretty good just with your mouth 'n' hands..." 

Luffy pulled away, and Zoro bit back a faint sound of disappointment. "Yeah, yeah! I wanna feel Zoro. You're not allowed to come yet." He tried to sound stern, like a captain giving an order, but it was too breathy, Luffy's eyes wide and a little wild with his own want. 

Zoro rolled to the side and dug through the supplies drawer. This _definitely_ called for lube, and a lot more of it than usual. "C'mere, Luffy. Let me get you ready." 

That was a whole new experience on its own, too. Luffy was _tight_ when he pressed his first finger in, and as he worked that beginning intrusion, he heard his captain panting softly as he worked to relax, worked to open up enough to let them move forward. That small body, so much less flexible than usual, so much less ready to stretch as far as anyone could need. It was weird. 

"You okay?" Zoro asked as he slid the second finger in, pressing a light kiss to Luffy's hair as he worked him slowly open. Luffy moaned, tiny and high, but nodded. Still eager. Still ready for this. 

Three fingers. Way more preparation than Luffy ever needed before. But he was moaning steadily now, arching himself for Zoro's touch and murmuring soft little sounds that didn't form words but fully communicated his need nonetheless. He really wanted this… 

Finally, Zoro thought he'd gotten Luffy as warmed up as he was ever going to get — and Luffy was starting to throw impatient looks at him, pleasure-hazed but imperative. "Ready?" 

"Ready!" It came out as the next thing to a chirp, and Zoro laughed, then rolled into a sitting position and gathered Luffy up into his lap. How easy it was to hold him one-handed, so small as he was — and what a feeling of control. There was normally no controlling Luffy ever, not even restraining him a little, but Zoro could sweep him easily like this. It felt… good. 

_Very_ good. 

Luffy was panting as Zoro started to press in, moving slow and biting his lip to control himself. _Shit_ , god, Luffy was so tight, even with all that preparation. Watching himself pierce into that tiny body, the way Luffy opened up so slowly around his cock, and feeling the way every quick breath made those muscles flare and lock down again… damn, that was good. Better than he'd expected. 

"Zoro…" Luffy murmured, tilting his head up and reaching out to stroke Zoro's face. "Feels so good, Zoro… you're so _big…_ " 

"Captain." Zoro kissed those reaching hands. "You tell me if you need me to stop, got it?" 

"Not gonna happen." Luffy grinned. "I'm gonna take all of you." 

And he did. Every moment, Zoro was half-expecting to find an end, to find the limit of Luffy's tolerance or this body's endurance, but he seated Luffy fully on himself, nearly trembling with the effort not to thrust into that heat, not to _move_ until he knew that little body could take it. Luffy's stomach showed a bulge, the press of Zoro's cock filling him up so completely, and both of them were panting, the harmony of their breaths filling the air. 

"C'mon," Luffy mumbled, and somewhere found it in himself to _wiggle_ in Zoro's lap, the motion shuddering pressure up and down his cock. "Move, Zoro." 

"If you're—" 

"Stop asking! Move!" 

That was his captain, all right. Caution and restraint were for other people. Slowly, Zoro rocked himself — moved Luffy more than his own hips, lifting the small captain slightly to draw out a little and then lowering him to thrust back in. 

_Like he's a toy. Like I can just — hold him, do that, move him how I want._ A new shudder of lust rocked him, and he repeated the motion, faster, as Luffy whined aloud. 

"Yes! Zoro, yes!" 

That was all he needed. In earnest, Zoro began to fuck upward into Luffy, both hands braced around his middle to lift and lower him, to _use_ his beloved alpha as a toy for his own pleasure. And Luffy rode right along with him, filling the air with a litany of happy cries and whines of pleasure. 

"Zoro, Zoro! Yes, oh — oh!" 

"Captain," Zoro whispered, head bowed. He couldn't let loose with the kind of noise Luffy did. For him, quiet was the way. "My captain. My alpha. You feel so good…" 

"Zoro!" Luffy was arching now, and his body clenched down around Zoro — close, he could feel it, Luffy was so close to coming, just needed a little more… 

Zoro had no knot to lock him, but he wrapped an arm around Luffy's middle, feeling his stomach bulge with every thrust, and reached down with his freed hand to curl a couple fingers around Luffy's erection, as shrunken as the rest of him — to squeeze his knot the way he knew Luffy needed. 

Luffy threw back his head against Zoro's chest and _howled_ as he came, and in the tight squeeze of rippling inner muscles, Zoro lost his grip on himself, lost the war against his own pleasure, and gasped as stars burst behind his eyelids. He thrust himself into Luffy to the hilt and stayed there, twitching as he filled his captain's shrunken body with his seed. 

He wasn't sure how long they stayed like that — pressed tight together though not locked, Luffy's tiny weight in his lap all but nothing. Slowly, the pleasure subsided, and Luffy gave a little squirm. 

"Hey, Zoro," he said softly. "I think I'm gonna go back to normal soon…" 

"Roger that," Zoro murmured in answer, and rolled them carefully onto their sides — sure enough, moments later, there was a soft pop of displaced air, and a very normal sized Luffy was curled up with Zoro, back pressed to Zoro's chest. 

"That's better," he said lightly, and then, "Hey, Zoro. That was fun, wasn't it?" 

"Yes," Zoro agreed. "That was a good idea, captain."

**Author's Note:**

> Come chat with me on my [Discord server](https://discord.gg/SWVYBBn)!


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